add flexibility tax and switch H2 subsidy by default

Created by: fschreyer

This adds a flexibility tax to REMIND main. Currently, the flexibility tax provides a subsidy to electrolysis with increasing VRE share. It is based on the idea that with high wind and solar shares, electrolysis can operate flexibly (at lower capacity factors) and face lower-than-average electricity prices. This first implementation reduces the capacity factor of elh2 from 0.9 to 0.5 and provides a subsidy that roughly halves the electricity price that electrolysis sees. This follows results of more detailed power system models. The flexibility subsidy is switched on by default.

In addition, the switch cm_FlexTaxFeedback can be switched on (it is off by default). This is work in progress and would enable the model to choose the capacity factor of electrolysis endogenously and includes a feedback of flexible operation of electrolysis on inflexible electricity demands. The lower-than-average electricity prices of electrolysis are balanced (load-weighted) by higher-than-average electricity prices for industry and buildings (tdels).